Harper was replaced by Nate Schierholtz in the top of the fourth inning after going 0-for-1 in the game.
Manager Matt Williams said that an illness has been going around the clubhouse recently. The Nationals won the game 4-2.
"He was a little lightheaded, not feeling great. We'll see how he is tomorrow," Williams said, according to MLB.com. "He may have a little bit of the gunk that everybody else has had."
Harper is hitting .270 with 13 homers and 32 RBI in 90 games this season. The Nationals have an 11 1/2-game lead over Atlanta in the National League East and can clinch the division with a victory Tuesday over the Braves.
- Scooby Axson